Experimental modeling of the formation of graphite and diamond as a result of carbide–fluid interactions was performed in the Fe3C–SiO2–Al2O3–(Mg,Ca)CO3 systems at 6.3 and 7.5 GPa and 1100–1650◦C. In the experiments with ƒO2-gradient (7.5 GPa, 1250–1350◦C), graphite + magnesiowüstite + garnet ± cohenite assemblage was formed. Graphite was produced through the redox interactions of carbide with carbonate or CO2 (reducing conditions), and redox reactions of magnesiowüstite and CO2 (oxidizing conditions). At 1450–1650◦C, crystallization of graphite, garnet, magnesiowüstite and ferrospinel, as well as generation of Fe2+,3+-rich carbonate–silicate melt occurred. This melt, saturated with carbon, acted as a medium of graphite crystallization and diamond growth on seeds. In the experiments without ƒO2-gradient (6.3 GPa), decarbonation reactions with the formation of CO2-fluid and Fe,Mg,Ca-silicates, as well as C0-producing redox reactions of CO2-fluid with cohenite were simultaneously realized. As a result, graphite (± diamond growth) was formed in assemblage with Fe2+,Fe3+,Mg-silicates and magnetite (1100–1200◦C), or with Fe3+-rich garnet and orthopyroxene (1300–1500◦C). It has been established that a potential mechanism for the crystallization of graphite or diamond growth is the oxidation of cohenite by CO2-fluid to FeO and Fe3O4, accompanied by the extraction of carbon from Fe3C and the corresponding reduction of CO2 to C0 .